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 Need help in examining a PCB scheme to get Phidget to read 4 Volts

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MartinT



Number of posts : 20
Registration date : 2007-12-19

PostSubject: Need help in examining a PCB scheme to get Phidget to read 4 Volts   7/3/2011, 19:42

Guys I am gonna need your help with this;

I wanted to get rid of needing the LPT port for pitlane sensing and lights, so I got me a Phidgets 0/16/16 IOBoard.

I have an SSD with powerbaseProSH connected to a PC, and I use IR lightbridge type sensors for my Pitbay detection. The PCB with components that I made is this one, I got the plan years ago of off a German Website. The thingy works flawlessly, the pin outputs (the blue lines in German "leiterbahn zum PC") put out 2,8 Volts. That was fine for my LPT port, but the Phidgetboard needs to see at least 4Volts to change from a 0 to a 1 logical state. (it takes 4-30V up to 2Amps)



So how do I get 4V out of the IC thingy?


The solution I came up with is to use a BC547 transistor. I could make something like this as in the plan below:
Where it reads Pin2,3,4,5 I will take the outputted signal from the board from above (the 2.8V ones remember...). (so that will stay fully intact, and all I need to do is make four (for every pit detector 1) things like below in the picture:
Instead of powering the red leds, I just input the 5V into the Phiget board so it sees >4V and voila it registers when there is a car in the pitbay!



Well that's the theory at least, but will this work, and do I need to put some resistors in somewhere, and if so why and what value?
(The red 10K resistor is used to shield the LPT port I guess, but there is no LPT port connected here, and the green 270 Ohm is used to limit the current flowing through the leds, but the way I will use it, there are no leds connected anyhow)

Well as you'll have noticed from my questions my electronic knowledge is very limited, I'm more like monkey see, monkey do, then I get by fine biggrin.gif)
So hopefully somebody can shine a light on this matter (and if there is another simpler solution, well bring it on of course! (like I tried to input 6 volts in the IC, and it output 3.3, so half a volt higher, but I did not dare to put in even higher voltage, since I don't know what the effect on the LDRs and leds will be, and I don't want to run the risk of frying anything other than an egg...(but if it can safely be done to like 8V, well that would be the easiest, since there is no other hardware (like the second picture) needed. So if someone can confirm that...


Thanks guys!

Martin

PS. of course I have seen the other plans in this forum for the detectors, but I reaaly want to keep the detectors as I have them now, since they work very well, and the LDR and leds are not accessible, since built in the track. (The Pcb part I can access easily)
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MartinT



Number of posts : 20
Registration date : 2007-12-19

PostSubject: Re: Need help in examining a PCB scheme to get Phidget to read 4 Volts   8/3/2011, 13:09

So I did some study and played around a bit with the BC547 transistor, but it worked only partially: (well it works, but not the way I hoped...)

I connected the +5V DC supply to the collector,
the base (blue line in layout) was hooked up to the +2,8V output of the detector,
the emitter went to the input of the Phidget board,
(and common gnd from the DC supply was connected to the gnd of the Phidget board)

So the layout looked like this:




(When there was a car on the detector, the IC from the LDR detector put out the 2.8Volt.)

But no input registered (so less than 4V) into the Phidget. However, when I touched the base (blue line in layout) with my finger, the Phidget registered a nice 5V.
It did not matter at all whether the base was or was not connected to the 2.8V line.
Every time I touched the base, the Phidget got 5V and went from a logical state 0 to 1. (which is the whole purpose of this deal...)

So I am still stuck with the problem of how to make the 2.8V signal into a 4V (or higher Voltage) signal

Help me pleaaase!! scratch
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slotmanege



Number of posts : 54
Localisation : Netherlands
Registration date : 2007-11-02

PostSubject: Re: Need help in examining a PCB scheme to get Phidget to read 4 Volts   8/3/2011, 18:05

Hi Martin,

Perhaps you can replace the LM324 with a "rail to rail" opamp.
The maximum output (voltage) of the LM324 = Vcc - 1,5 Volts... so would be max of 3,5 Volts.
With a "rail to rail" opamp it is possible to go to (nearly) the complete input voltage rails. And you would not need to amplify the output anymore.

To my opinion you use the BC547 as if it were a digital building block (signaled by voltages)... but BC547 is analog.
Just by hooking voltages on the BC547 does not do anything. You have to make sure there is a current. That is exactly what happens when you touch the base line... the current flows via your finger!


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MartinT



Number of posts : 20
Registration date : 2007-12-19

PostSubject: Re: Need help in examining a PCB scheme to get Phidget to read 4 Volts   8/3/2011, 19:23

Hi Slotmanege, thanks for your answer and thoughts!

However after some investigating, I decided to increase the Voltage into the detector IC (8V)
and now it outputs a nice 4 Volts! So I don't need any other transistor hardware anymore, problem solved. (well of course I have to prove it over the coming days of use;-)

Best regards,
Martin
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slotmanege



Number of posts : 54
Localisation : Netherlands
Registration date : 2007-11-02

PostSubject: Re: Need help in examining a PCB scheme to get Phidget to read 4 Volts   9/3/2011, 13:31

Hi Martin,

Yes, that solution is probably the easiest way and good enough! cheers

Regards,
Danny
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